This invention relates to a control system for internal combustion engines, which is provided with an exhaust gas recirculation system, and more particularly to a control system for internal combustion engines, which has a fail safe function of detecting a failure in the control section of the exhaust recirculation system and perform necessary actions for safety.
An exhaust gas recirculation system (EGR system) is already known which includes an exhaust gas recirculation passageway communicating the exhaust system of an internal combustion engine with the intake system of the same for introducing inert exhaust gases in the exhaust system into the suction mixture in the intake system therethrough to lower a maximum combustion temperature in engine cylinders and accordingly decrease the amount of NOx to be present in the exhaust gases.
On the other hand, an internal combustion engine is conventionally provided with various control systems for controlling the engine in order to operate in optimum states. For instance, air/fuel ratio control systems have been proposed e.g. by the assignee of the present application, which perform feedback control of the air/fuel ratio of an air/fuel mixture being supplied to the engine, in response to the concentration of an exhaust gas ingredient emitted from the engine. Such control systems are connected with various sensors for detecting various operating conditions of the engine to control the air/fuel ratio and other engine operating factors in response to output signals of these sensors so as for the engine to operate in optimum states.
Exhaust gas recirculating operation by means of the aforementioned exhaust gas recirculation system needs to be interrupted when the engine is under particular operating conditions such as engine idle, engine deceleration and low engine temperature operation for prevention of incomplete combustion within engine cylinders. To this end, an exhaust gas recirculation system in general is connected to a control system which is one of the aforementioned control systems and is controlled by a control signal supplied from an electronic control circuit or the like provided in the control system to interrupt its exhaust gas recirculating operation during the above-mentioned particular engine operating conditions.
The conventional exhaust recirculation systems include a type which comprises an exhaust gas recirculation control valve having negative pressure-actuatable means and arranged across the exhaust gas recirculation passageway, a negative pressure passageway communicating the negative pressure-actuatable means with the exhaust gas recirculation control valve, and an EGR control valve formed of a solenoid valve arranged for opening and closing the negative pressure passageway.
The solenoid valve which forms the EGR control valve is arranged to have its solenoid energized for operation by means of a power transistor provided within the above-mentioned electronic control circuit or the like. There can occur a break or a short in the solenoid winding or in the wiring related to the solenoid winding and the power transistor, or a failure in the power transistor per se. In such an event, control of the exhaust gas recirculation cannot be properly effected, which can cause incomplete combustion in engine cylinders, thus making it impossible to control the engine to operate in optimum states. Particularly in an air/fuel ratio control system, if a control device, which is used to control the exhaust recirculation control valve, becomes defective, the air/fuel ratio can be controlled to an improper value, which can spoil the driveability and exhaust emission characteristics of the engine.